This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Site-directed spin labeling (SDSL) is a powerful tool for monitoring the structure and dynamics of both soluble and membrane proteins. Measurements of the spectral properties of the paramagnetic nitroxide probe with electron paramagnetic resonance (EPR) spectroscopy provide a wealth of information on the environment in the protein. The HIV-1 protease is a 22 kDa homodimeric protein essential for function of the AIDS virus, and protease inhibitors have been developed into effective HIV drugs. We are particularly interested in the details of HIV-1 protease flap dynamics that can be learned from multifrequency SDSL EPR studies. Earlier SDSL studies of several spin labels at 9.5 GHz do not reveal any significant change in the flap dynamics in the presence or absence of protease inhibitor, contrary to expectration. Preliminary results at 170 GHz do indicate small but discernible differences in the flap dynamics in the presence or absence of inhibitor with the inhibited form showing clear evidence of slower tumbling. Based on these promising initial results we are refining the experimental design in order to enhance the spectral differences. We suggest that complementary studies at 240 GHz will further resolve the spectral sensitivity to the observed small differences in dynamics and will provide a useful means for quantifying the response of HIV-1 protease to the presence of various inhibitors.